Pleuromutilin (also now known as pleuromulin) is an antibiotic produced by cultures of the basidiomycete Pleurotus mutilis. The antibiotic has antibacterial activity. A group of new derivatives of pleuromutilin having antibacterial activity is described in German Offenlegungschrift No. 2,248,237 (Apr. 12, 1973). One of these derivatives in particular, 14-dioxy-14-[(2-diethylaminoethyl)mercaptoacetoxy]mutilin (known generically as tiamutilin or tiamulin), as the hydrogen fumarate salt, has been found to be very active against microorganisms such as Streptococci, Staphylococci and Mycoplasmas and particularly useful in veterinary medicine. See Antimicrobial Agents and Chemotherapy, May 1975, pages 507-516, 517-521. This substance, however, causes difficulties in manufacture, formulation and administration. Research on the substance has now demonstrated that the formation of the neutral fumarate salt provides a material at least equally active on a weight basis but which has markedly different physical properties which overcome the more serious disadvantages of the hydrogen fumarate.